1. Field of the Invention
The present invention relates to fuel delivery systems. More specifically, the present invention relates to systems and methods for delivering liquified gas from a holding tank to an engine.
2. Present State of the Art
The increasing output of automobile emissions and the decreasing supply of oil reserves has motivated the search for alternative motor vehicle fuels. One alternative fuel is natural gas. Natural gas is clean burning and can be stored in a dense, high energy liquid form. Liquefying natural gas is accomplished by cooling the natural gas to a cryogenic temperature, typically below xe2x88x92260xc2x0 F., which condenses the gas into a liquid. Working with and keeping natural gas at a cryogenic temperature, however, creates inherent problems. Furthermore, natural gas, prior to combustion, is a harmful greenhouse gas. As such, it is important that the escape of any natural gas be minimized to prevent increased harm to the atmosphere.
In one approach to using natural gas in automobiles, the natural gas is initially stored in large tanks at refueling stations. The large tanks maintain the fuel at a cryogenic temperature so as to keep the natural gas in a dense liquid state. Smaller insulated fuel tanks are located within the automobiles and can be filled with the liquified natural gas at a refueling station. As discussed above, it is desirable to store the natural gas in a liquified state. It is also beneficial, however, to have the automobile fuel tank sufficiently pressurized so that the fuel therein will automatically flow to the vehicle engine. Although a pump can be used to deliver the fuel to the engine, use of a pump requires energy. Furthermore, pumping natural gas at cryogenic temperatures has been found to be problematic.
In one approach to obtaining the desired pressure within the automobile fuel tank, systems have been incorporated into refueling stations which warm the liquified natural gas as it is pumped into the automobile fuel tank. By heating the liquified natural gas to a desired temperature, a portion of the liquified natural gas vaporizes within the fuel tank to produce the desired pressure. The pressure created within the fuel tank as a result of warming the fuel is called xe2x80x9csaturation pressure.xe2x80x9d Although this process achieves the desired objective, it also produces several problems.
For example, the systems for heating the natural gas at the refueling station are time consuming and expensive to operate and build. Furthermore, as a result of warming the natural gas, less natural gas can be stored within the fuel tank. In addition, since all of the natural gas that is pumped into the automobile fuel tank is heated, the fuel must be used relatively quickly to prevent having to vent any of the natural gas to the atmosphere. Although the automobile fuel tank is insulated, once the liquified natural gas is pumped therein, the fuel begins to slowly warm towards an equilibrium with the outside temperature. As the fuel warms, the pressure within the tank increases. Once the tank reaches a designed relief pressure, a pressure relief valve is opened allowing a portion of the natural gas to escape into the atmosphere, thereby decreasing the internal pressure. The time period that a tank can hold natural gas without having to vent is called the xe2x80x9chold time.xe2x80x9d As previously discussed, releasing natural gas into the atmosphere is both wasteful and potentially harmful.
In contrast, if the natural gas is consumed too quickly, the pressure within the fuel tank can drop below the required operating pressure. As liquified natural gas is consumed, the volume of the vapor holding portion of the fuel tank is increased. As this volume increases, a portion of the liquified natural gas is vaporized to fill the space within the fuel tank. Vaporization of natural gas is an endothermic process which absorbs heat. Accordingly, as the natural gas within the fuel tank is vaporized, the temperature and thus pressure within the fuel tank decreases. If liquified natural gas is consumed too quickly, the pressure will drop below the operating pressure.
In an alternative approach to pressurizing the automobile fuel tank, a heater is directly coupled with the automobile fuel tank for heating the liquified natural gas therein. The problem with this approach is that it takes both time and energy to heat the fuel within the fuel tank. Furthermore, the same problem exists of having to use the natural gas relatively quickly to prevent having to vent portions of the natural gas to the atmosphere.
Other problems in conventional liquified natural gas systems relate to the lines extending from the fuel tank to the engine. Many of the prior art systems require the use of electronic switches, solenoids, and computers to operate them. The use of such electronics is expensive, increases the complexity of the system, decreases the reliability of the system, and consumes large amounts of energy.
The same problems as discussed above for vehicles are also applicable to using natural gas or other liquified gases to run engines that are not vehicle related.
It is an object of the present invention to provide improved delivery systems and methods for delivering liquified gases to an engine.
Another object of the present invention is to provide improved delivery systems which do not require a liquified gas to be warmed as it is transferred from a refueling facility to a holding tank for operating an engine.
Yet another object of the present invention is to provide delivery systems which do not require all of the liquified gas disposed within the holding tank to be warmed therein.
Still another object of the present invention is to provide delivery systems which significantly increase the hold time of the liquified gas in the tank.
A further object of the present invention is to provide delivery systems for liquified gas which maintain a desired pressure within the holding tank substantially independent of the gas consumption rate.
Yet another object of the present invention is to provide delivery systems which enable relatively quick pressurization of the tank holding the liquified gas.
Finally, an additional object of the present invention to provide delivery systems which provide fuel lines extending from the tank to the engine that do not require the use of electronic switches, solenoids or computers to function.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a liquified gas delivery system and method are provided for use with an engine mounted on a mobile or stationary vehicle. The liquified gas delivery system includes an insulated holding tank configured to receive a liquified gas at saturated liquid/gas conditions. The holding tank bounds a chamber which includes a liquid holding portion for holding liquified gas and a vapor holding portion for holding liquified gas vapor. A vapor conduit extends from the vapor holding portion of the tank to a valve device such as an economizer valve or ecoshunt valve. A liquid conduit extends from the liquid holding portion of the tank to the valve device. A transition conduit extends from the valve device to a vaporizer.
The valve device is configured to operate in one of two positions depending on the pressure within the vapor holding portion of the tank. When pressure within the vapor holding portion of the tank is below a select pressure, the valve device facilitates the flow of the liquified gas from the tank to the vaporizer. When the pressure within the vapor holding portion of the tank exceeds the select pressure, the valve device blocks the flow of liquified gas and facilitates the flow of the liquified gas vapor from the tank to the vaporizer. Once sufficient liquified gas vapor has been removed from the tank to drop the pressure therein below the select pressure, the valve device again facilitates the flow of the liquified gas from the tank to the vaporizer.
The vaporizer is heated with coolant from the engine. As liquified gas is passed through the vaporizer, the elevated temperature causes the liquified gas to flash into a vapor. A delivery conduit extends from the vaporizer to the engine for delivering the liquified gas vapor thereto. A return conduit having a check valve coupled therewith extends from the delivery conduit to the vapor holding portion of the tank. Feeding of the liquified gas vapor from the return conduit to the vapor holding portion of the tank functions to pressure the tank.
It is desirable to keep the liquified gas within the tank at the lowest economical temperature. At such a temperature, however, there may be insufficient saturation pressure within the vapor holding portion of the tank to drive the liquified gas from the tank to the engine. Until such time that the liquified gas warms up from the outside environment to a point that it produces the required saturation pressure, the liquified gas vapor feeding from the return conduit to the vapor holding portion of the tank functions to create the required pressure to operate the system.
To enable effective pressurization of the tank using the return conduit, the vaporizer is most effective when positioned a required distance below the surface of the liquified gas in the tank. Specifically, the head between the surface level of the liquified gas and the point in the vaporizer where the liquified gas is vaporized must be sufficiently large to create a required pressure on the vaporized gas leaving the vaporizer. This required pressure must be greater than the summation of the pressure losses on the gas as it passes from the tank through the valve device, vaporizer, and back to the tank. As a practical matter, to enable operation of the engine at low levels of fuel within the tank, the vaporizer is preferably positioned below the elevation of the tank.
The systems of the invention have several advantages over prior conventional systems. For example, in the present inventive system the liquified gas within the holding tank can be maintained at its lowest possible temperature. As a result, it is not necessary to incorporate systems for warming the liquified gas as it is transferred from a refueling facility, or systems for warming the liquified gas within the tank. Furthermore, since the liquified gas is maintained at a low temperature, the hold time for the tank is much longer than conventional systems. In addition, the present system can continually regulate the pressure within the tank independent of the consumption rate. Finally, the system can be operated in a passive configuration which does not require the use of electronic solenoids, switches, or computers to run.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.